A spectrophotometric study of neodymium(III) complexation in sulfate solutions at elevated temperatures

The formation constants of neodymium complexes in sulfate solutions have been determined spectrophotometrically at temperatures of 30-250 degrees C and a pressure of 100 bars. The dominant species in the solution are NdSO4+ and Nd(SO4)(2)(-), with the latter complex being more important at higher temperature. Equilibrium constants were calculated for the following reactions: Nd3+ + SO42- = NdSO4+, beta(-)(1), Nd3+ + 2(.)SO(4)(2-) = Nd(SO4)(2)(-), beta(2); NdSO4+ + SO42- = Nd(SO4)(2)(-), K-s. The values of beta(1) and beta(2), were determined for 30 and 100 degrees C, whereas for higher temperatures it was only possible to determine the stepwise formation constant Ks. The values of the formation constants obtained in this study for 30 and 100 degrees C are in excellent agreement with those predicted theoretically by Wood [Wood, S.A., 1990b. The aqueous geochemistry of the rare-earth elements and yttrium. 2. Theoretical predictions of speciation in hydrothermal solutions to 350 degrees C at saturation water vapor pressure. Chem. Geol. 88 (1-2), 99-125] and Haas et al. [Haas, J.R., Shock, E.L., Sassani, D.C., 1995. Rare earth elements in hydrothermal sysytems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures. Geochim. Cosmochim. Acta 59 (21), 4329-4350], and those for the stepwise formation constant (K,) agree reasonably well with the predictions of Wood (1990b). (c) 2005 Elsevier Inc. All rights reserved.